The present invention is the fruit of the development of a Bus Supplayer device, that is to say a voltage regulator for feeding a Bus which is portable with some standards: SCSI (Small Computer System Interface), SCSI-2, SCSI-3, as well as provided with at least nine active termination with high precision (110.OMEGA. at 2%), although the description hereinbelow is made with reference to this specific field of application with the only purpose of simplifying the illustration.
A device of the above-mentioned type essentially operates as a voltage regulator and feeds every line of the Bus via a so called active termination comprising a matching impedance with a switch associate thereto. A voltage regulator and the active termination (switch+resistance) are formed inside the integrated circuit.
A regulator of known type is schematically illustrate in FIG. 1. In conditions of operation, the regulator feeds every line of the bus with a matching impedance of high precision; in alternative, if required by the CPU that manages the Bus, the regulator can set the terminations into a state of high impedance disconnecting itself from the Bus. Hence the output of the regulator is interfaced with the Bus by means of a switch that is usually formed with a MOS transistor connected in series to a respective resistor for every line of the Bus.
A conclusive condition for the efficiency of the regulator is the precision of the matching impedance, that corresponds to the resistance "seen" by the output pin of the regulator connected to the line of the Bus in direction of the inside of the integrated circuit. EQU R.sub.TERM =R.sub.DS.sub..sub.-- .sub.MOS +R (1)
The termination connects to the output of the regulator, which represents a node at low impedance.
The precision of the resistance R.sub.TERM must be further maintained in presence of potential changes in the power supply, in temperature, and also with respect to the fluctuations of the voltage of the bus line between V.sub.IN and GND.
In the past, for SCSI applications the regulators of this kind were always integrated and formed interfacing them with each one of the Bus lines via resistors, and possibly switches, external to the integrated circuit. Obviously this created serious problems of overall circuit dimensions, as well as higher costs and management burden of the regulator Bus system.
In FIG. 2 an example of a regulator formed according to this approach proposed by the prior art is shown.
A further method for obtaining a resistive termination with high precision, internal to the integrated circuit incorporating the bus, is that of using a technique known as "Laser Trimming". Essentially, during the EWS (Electrical Wafer Sort) test phase of the integrated circuit, the resister connected in series to the MOS switch undergoes a trimming action by a laser beam in such a way to calibrate the resistive layer up to a desired value.
It is clear though that such a known solution needs an engineered system comprising the laser trimming and the technology for creating a device able to stand the trimming action.
In FIG. 3 a schematic view of a resistive termination cell is shown, which undergoes a trimming action by means of the laser trimming technique.